1. Field of the Disclosure
This invention pertains to a method and apparatus for preparing a printing form from a precursor, particularly a method and apparatus for preparing the printing form by thermally treating a photosensitive precursor.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photo-polymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the meltable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 and U.S. Pat. No. 6,797,454 each describe an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element with a web of absorbent material. Both apparatuses include a drum for supporting the photosensitive element during thermal development. Both apparatuses include a hot roll for supporting the web of absorbent material in contact with the photosensitive element to heat the element and remove the molten polymer with the absorbent material during thermal development. The hot roll can be metal having an outer surface coated with an elastomer layer with a Shore A hardness units between about 30 and about 90. The elastomer can be silicone rubber. Alternatively, the hot roll can be aluminum with an anodized finish, without any elastomer layer. The hot roll is heated by an electrical core heater to provide a temperature sufficient to melt a portion of the composition, but can be heated by steam, oil, hot air, and a variety of other heating sources to provide a temperature sufficient to melt the composition. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent material. As the drum and hot roll rotate in contact together, the web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and is then separated from the element. Several cycles of passing the element past the hot roll are repeated to progressively remove the unirradiated composition from the printing element. The hot roller is coated with a layer of elastomer coating that can have a Shore A hardness between about 30 and about 90. A preferred coating is silicone rubber about 3/16 inch thick.
In two embodiments of a thermal processor, commercially identified as CYREL® FAST 1000TD and CYREL® FAST TD4260 processors, the hot roller is a metal roller that is heated with an electrical core heater. The hot roller in the CYREL® FAST 1000TD originally included an exterior layer of a solid silicone rubber having a Shore A hardness of about 50, but the rubber covering was eliminated early on due to problems with durability as the rubber covering tended to delaminate and/or harden with use. In general, solid silicone elastomers are not compressible and have a Poisson's ratio generally above 0.45, and typically nearly 0.5.
A problem sometimes arises with thermal processing that it can be difficult to maintain the base support of the element below a temperature at which the base support distorts and/or shrinks while the composition layer is heated to a temperature sufficient to melt the unirradiated composition for removal in depth. Distortion and/or shrinkage of the base support can be avoided or at least reduced if the temperature the base support is maintained below its glass transition temperature, Tg, and/or below its annealing temperature that occurs during its manufacture. Despite efforts to cool the surface of the drum on which the photosensitive element resides, the drum typically includes an elastomeric outer layer which can isolate the base support from chilling or cooling by the drum. Base supports which experience temperatures at or above the glass transition temperature and/or annealing temperature of the base support for the element can result in distortion or deformation of the resulting printing form or plate. Deformations include waves of localized distortions resulting in a non-planar topography of the photosensitive element. Waves of distortions can form in different locations in each element processed.
Furthermore, the base support can be deformed during thermal development by the force of the hot roller that carries the absorbent material into contact with recessed portions in conjunction with the photosensitive element residing on some types of elastomeric layer on the drum. Oftentimes, in order to achieve improved relief uniformity the pressure at the nip between the hot roll carrying the absorbent material and drum carrying the photosensitive element can be increased to impress the absorbent material into the recessed areas. Increased nip pressure on the element increases the residence (i.e., dwell) time for the transfer of heat to the element. The resilient surface provided by some types of elastomeric layer under the photosensitive element can result in a longer nip zone as well as flexing or bending of the base support of the element as a result of the surface deflecting under the pressure exerted by a hot roller. Increased residence time that raises temperatures at or above the glass transition temperature or its annealing temperature of the base support, particularly while flexed, can also result in the distortion or deformation of the resulting printing form or plate. Deformation of the base support can lead to image dependent waves and creases in the resulting printing form or plate. The problem can be exacerbated by photosensitive elements with thick photopolymerizable layer as greater depth of relief surface is needed. The relief depth for the thick printing forms can be as much as about half of the thickness of the photopolymer layer.
Additionally because the photosensitive element is essentially a nearly incompressible layer, application of significant force by the hot roller at the nip can result in mismatch between the transport speed of photosensitive element on the drum and the absorbent material carried by the hot roll that are image dependent and sensitive to frictional changes. Image dependency is due to the increasing difference in height between the image areas that are raised portions and non-image areas that are recessed portions as thermal development of the relief surface progresses. Mismatch of transport speed creates differences in sheer forces that can lead to image dependent creasing in the support base that is adhesively attached to the photopolymerizable layer of the photosensitive element or impact dimensional control of the base support.
Relief printing forms having distortion's in the base support and/or the cured photopolymeric layer also result in poor print performance. In multicolor printing, when one or more of the relief printing forms have distortion the printed image has poor registration. Even in single color printing, distortion in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. The relief printing form having distortion's may also incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.
It is thus desirable to provide a method for preparing a relief printing form from a photosensitive element and an apparatus for thermal development that reduces or limits heating of the base support of the element, and provides for maintaining the base support below a temperature at which the base support distorts and/or shrinks while the composition layer is heated to a temperature sufficient to melt the unirradiated composition for removal in depth. It is also desirable for the method and apparatus to avoid or reduce problems associated with delamination of an elastomeric covering on the hot roll, the need for significant force applied by the hot roll at the nip, and mismatch of transport speeds between the hot roll and the photosensitive element on the drum. Such that the desired method and apparatus can eliminate or reduce the tendency for deformations, distortions, and/or shrinkage of the base support of the photosensitive element, and/or for waves and/or creases in the resulting printing form.